The ever increasing dependence on limited fossil fuel resources and resulting pollution have created an urgent need for cleaner fuel sources in pursuit of a more secure energy future. One alternate fuel source is hydrogen. Unlike fossil fuels, which produce energy by combustion and yield polluting byproducts, hydrogen is consumed by chemical reaction with oxygen forming only water as a byproduct. Thus, hydrogen has enormous potential as a clean power source for future generation of automobiles.
Automobiles require a pre-specified minimum mass of hydrogen to run for long distances before refueling. Calculations reveal 7 kg of hydrogen are used to run a vehicle for about 300 miles. The density of hydrogen at atmospheric temperature and pressure is 0.083 g/L while at a pressure of 700 bars it is 39.6 g/L. The 700 bar limit is set by the pressure rating for high-strength, lightweight carbon-fiber composite tanks. Further research is required to increase this limit by strengthening composite fibers and ensuring impermeability to hydrogen gas. The aforementioned density value means 7 kg of hydrogen gas at 700 bars would occupy about 220 liters. Such a volume is far too large to be stored on-board a vehicle. To reduce the volume to an acceptable level, hydrogen can be stored in liquid form at cryogenic temperatures. The density of liquid hydrogen at 20.3 K is 70.8 g/L, roughly twice the density of compressed hydrogen at 700 bars. Thus, cryogenic storage would use about 100 liters to store 7 kg of liquid hydrogen. Two disadvantages of liquid hydrogen storage are high-energy consumption, associated with the liquefaction process, and continuous boil-off during storage due to the difficulty in thermally insulating the liquid hydrogen at such low temperatures.
Another promising and practical alternative to high-pressure gas storage and cryogenic liquid storage is the use of metal hydrides. Many metals (M) and alloys can react reversibly with hydrogen to form metal hydrides.M+0.5×HxMHx+heat
The hydriding (forward) process of the reaction absorbs the hydrogen and releases heat, while the dehydriding (reverse) process requires heat input to release the hydrogen. The hydriding process is the process that occurs while filling the vehicle with hydrogen at the filling station. The dehydriding process occurs when the hydrogen stored in the metal hydride is de-absorbed to be used in the fuel cell for power production. The rates of both the hydriding (charging) and dehydriding (discharging) processes are highly dependent on temperature, i.e., they are kinetics-driven. The hydriding process preferably includes quick removal of the heat generated by the reaction for the process to proceed. If the heat is not removed efficiently, the temperature would rise to a level that can stall the reaction. This temperature limit is different for different metal hydrides and is about 80° C. for metal hydrides currently being tested in automotive research. The dehydriding process requires heating the metal hydride to a temperature that depends on chemical thermodynamics. The dehydriding process cannot occur without heating.
In the vehicle, heat is generated while charging the metal with hydrogen at the filling station. The 2015 target for refueling time is less than 5 minutes. In order to achieve such a fast refueling rate, it is helpful that the high-rate heat generation associated with the fast hydriding process be removed efficiently. Removing the heat is even more challenging with faster refueling rate (i.e., shorter refueling time) because of the greater rate of heat generation. Subsequent release of hydrogen from the metal hydride for fuel cell use is achieved by heating to a specific temperature.
The volumetric density of metal hydrides (volume occupied by hydrogen per unit volume of metal hydride) is comparable to that of liquid nitrogen. But a major disadvantage of metal hydrides is low gravimetric density (mass of hydrogen stored per unit mass of metal hydride). Hence, a heat exchanger that occupies a small volume and provides as much of the available storage space for the metal hydride is required to quickly and efficiently remove the heat as it is generated by the hydriding process. The heat exchanger should also allow for thermal expansion of the metal hydride at higher temperatures.
The operating pressure of metal hydrides is directly related to temperature. At a given temperature, the operating pressure should be higher than an equilibrium pressure for the hydriding process to occur. The equilibrium pressure depends on the temperature and thermodynamic properties of the metal hydride. Increasing the operating pressure increases the temperature limit above which the hydriding process stalls. Exceeding the equilibrium pressure corresponding to the afore-mentioned 80° C. temperature requires metal hydride operating pressures in the range of 400-500 bars. Hence the heat exchanger should be designed to withstand such high pressures.
Metal hydrides can be available in powder form, or formed into pellets of any desired shape. Hence, the heat exchanger in a hydrogen vehicle is a storage device for metal hydrides that can provide sufficient cooling at high pressures to maintain temperature levels that render the hydriding and dehydriding processes highly efficient.